1. Field of the Invention
This invention relates to abrasive articles utilizing a binder which secures abrasive grains to a backing sheet, on fibers of a fibrous mat, or in a shaped mass, and to methods of making such articles utilizing a binder precursor solution including a reactive diluent. The reactive diluent reduces emissions of volatile organic compounds from the binder precursor solutions and, in some embodiments, renders the cured binder more flexible.
2. Discussion of the Art
Abrasive articles may be categorized as coated, bonded, and nonwoven abrasives. Coated abrasives generally comprise a flexible backing upon which an abrasive coating comprising abrasive grains and a binder is attached. The backing can be selected from paper, cloth, film, vulcanized fiber, etc. or a combination of one or more of these materials, or treated versions thereof. The abrasive grains can be formed of flint, garnet, aluminum oxide, alumina zirconia, ceramic aluminum oxide, diamond, silicon carbide, etc. Binders commonly comprise cured versions of hide glue or varnish, or one or more resins such as phenolic, urea-formaldehyde, melamine-formaldehyde, urethane, epoxy, and acrylic resins. Phenolic resins include those of the phenol-aldehyde type.
Nonwoven abrasive articles typically comprise a fibrous mat of fibers which have on at least a portion of their surface an abrasive coating comprising abrasive grains and a binder. The fibers can be formed from various polymers, including polyamides, polyesters, polypropylene, polyethylene, and various copolymers. Naturally occurring fibers such as cotton, wool, bast fibers and various animal hairs may also be suitable.
Coated and nonwoven abrasives may employ a "make" coating of binder precursor solution, which includes one or more of the above-named resins, in order to secure the abrasive grains to the backing when the resin is cured as well as to orient the abrasive grains on the backing or throughout the lofty fibrous mat. A "size" coating of resinous binder material can be applied over the make coating and abrasive grains in order to firmly bond the abrasive grains to the backing or fibrous mat. The resin of the size coating can be the same as the resin of the make coating or a different material.
In the manufacture of coated and nonwoven abrasives, the make coating and abrasive grains are usually first applied to the backing or lofty fibrous mats, the make coating partially cured, then the size coating is applied, and finally the make and size coatings are fully cured.
Generally, binders which include thermally cured resins provide abrasive articles having excellent properties, e.g., heat resistance. In order to render the resin precursors coatable, obtain the proper coating viscosities, and obtain defect free coatings, solvent is commonly added to the uncured resins.
When polyester or cellulose backings or lofty fibrous mats of such fibers are used, curing temperature is sometimes limited to about 130.degree. C. At this temperature, the long cure time along with solvent removal necessitate the use of festoon curing areas. Disadvantages of festoon curing areas include the emission of volatile organic compounds (VOC) such as organic solvents, unreacted resin precursors such as phenol and formaldehyde, and the like.
Of the many thermally curable resins, including phenolic resins, urea-aldehyde resins, urethane resins, melamine resins, epoxy resins, and alkyd resins, phenolic resins are used extensively to manufacture abrasive articles because of their thermal properties, availability, low cost, and ease of handling. Although phenolic resins are discussed herein, it should be appreciated by those skilled in the art that the principles discussed herein are applicable to other thermally curable resins, such as those previously named. The monomers currently used in greatest volume to produce phenolic resins are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butyl-phenol, p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A (bis-A or 2,2-bis(4-hydroxyphenyl) propane), are employed in smaller quantities for applications requiring special properties.
There are two basic types of phenolic resins: resole and novolak phenolic resins. Molecular weight advancement and curing of resole phenolic resins are catalyzed by alkaline catalysts. The molar ratio of aldehyde to phenolic is greater than or equal to 1.0, typically between 1.0 and 3.0. In the production of adhesive coatings for nonwoven and coated abrasives, one standard starting phenolic resin composition is a 70% solids condensate of a 1.96:1.0 formaldehyde:phenol mixture with 2% potassium hydroxide catalyst added based on weight of phenol. The phenolic resin composition is typically 25-28% water and 3-5% propylene glycol ether, which are required to reduce viscosity of the resin. Before this resin is used as a component of a make or size coating, i.e., to make it coatable, further viscosity reduction is often achieved using VOC. A binder precursor solution containing a phenolic resin which is used to produce a make coating may contain up to 40% of a VOC such as isopropyl alcohol to reduce viscosity and make the phenolic resin compatible with resin modifiers (flexibilizers), while a binder precursor solution which is used to produce a size coating might contain up to 20% of a VOC such as diethylene glycol ethyl ether. Unreacted phenol and formaldehyde in the final, cured resin also contribute to VOC.
In order to reduce emissions of VOC, efforts have been made to increase the water compatibility of phenolic resins. Fisher, in a review article without references titled "Water Compatible Phenolic Resins" in Proceedings of the American Chemical Society, Division of Polymeric Material: Science and Engineering; 65 pp. 275-276 (1991), describes currently known methods of making "water compatible" phenolic resins, their benefits, and their shortcomings.
Unfortunately, the water tolerance of phenolic resins suffers in many of the formulations designed to reduce VOC. "Water tolerance" refers to the measurement of the maximum weight percent of distilled water, based on initial resin weight, which can be added to a stirred, uncured phenolic resin via titration to begin causing visual phase separation (as evidenced by a milky appearance) of the resin/water mixture into aqueous and organic phases. As mentioned by Hoy et al, below, it is imperative that during the drying stage, after a particular substrate has been covered with a layer of coating, that a single phase be maintained until the water has evaporated away leaving the now insoluble organic polymer deposit. Another problem with many water compatible phenolic resins is that gel time is increased. "Gel time", as used herein, refers to the length of time at a given temperature that a phenolic resin transforms from a liquid to a gelled state. It is an indication of the rate of cure of a phenolic resin under established conditions.
Phenolic resins suitable for use in the manufacture of abrasive articles may optionally contain plasticizers, crosslinking aids, or other modifiers. Modifiers have been used to overcome deficiencies of phenolic resins in certain applications such as brittleness in the cured state and lack of water tolerance in the uncured state. Modifiers have previously been used to adjust the physical properties of the finished product, such as hardness in a wet environment, but many have required additional VOC for viscosity reduction.
It is known that nitroalkanes and urea react with formaldehyde, but these compounds have not been used as reactive diluents in the production of abrasive articles to the inventors' knowledge. It is also known that poly(oxyalkylene) amines react with phenolic resins, and there have been attempts by assignee to commercialize nonwoven abrasive articles employing poly(oxyalkylene) amines having molecular weight of 400 and above. These attempts were largely unsuccessful. The high molecular weight poly(oxyalkylene) amines typically require an organic solvent to render coatable the binder precursor solution in which they are mixed.
U.S Pat. No. 4,571,413 to Dolden et al. describes the use of polyethers [poly(oxyalkylene) diamines] in the preparation of modified phenolic resins for use in fiber-reinforced composite materials and phenolic foams. Improved impact strength and flexural properties are noted. 2-45 parts of polyether per 100 parts aqueous phenolic resin are suggested. All resole resins were acid catalyzed. There is no mention of base-catalysis, applicability to abrasive compositions, or lowering of VOC.
U.S. Pat. No. 4,786,683 to Schloman, Jr. et al. describes modified guayule resins containing poly(oxyalkalene) amines and phenolic resins for use as rubber modifiers.
U.S. Pat. No. 4,163,030 to Banucci et al. describes blends of polyetheramide-imide compounds and phenolic resins. Applicability to solventless dry powder coatings and electrical insulation is noted.
U.S. Pat. No. 3,734,965 to Becker describes poly(oxyalkalene) compounds with aldehydes and substituted phenols as curatives for epoxy resins.
U.S. Pat. No. 4,226,971 to Waddill e al. describes an epoxy curing agent derived from the phenol-aldehyde condensation product with the aminoalkylene derivative of a poly(oxyalkalene) polyamine.
U.S. Pat. No. 3,933,936 to Smith et al. describes aziridine-modified phenolic resins with good bond to wood, metal, ceramics, and plastics, and which have fast cure. There is no mention of abrasive applications, poly(oxyalkalene) compounds, or reduction of VOC.
U.S. Pat. No. 4,650,838 to Das et al. describes aromatic phthalocyanine compounds as modifiers for phenolic resins. Improved thermal stability and applicability to friction materials are noted. Novolaks or resolated novolaks are the focus.
U.S. Pat. No. 5,041,481 to Sugimori et al. describes amino compound adhesion promoters for curable compositions such as paints, adhesives, or sealing compounds.
U.S. Pat. No. 4,102,866 to Speranza et al. describes poly(oxyalkalene) compounds as a curative for epoxy-novolak resins.
U.S. Pat. No. 4,164,520 to Waddill et al. describes a process employing poly(oxyalkalene) compounds to accelerate the cure of epoxy resins.
British Patent No. 1,501,331 to Minnesota Mining and Manufacturing Company describes the use of poly(oxyalkalene) compounds with epoxy resins to manufacture friction materials.
U.S. Pat. No. 4,906,774 to Speranza et al. describes the preparation of urea-linked diamine product from poly(oxyalkalene) diamines and diisocyanates.
U.S. Pat. No. 4,154,724 to Schulze describes the preparation of ureido-functional poly(oxyalkalene) compounds.
U.S. Pat. No. 5,039,759 (Hoy, et al.) discloses the use of reactive urea, thiourea, and carbamate derivatives as cosolvents and reactive diluents for modifying water dispersible resins including polyester alkyd resins, carboxylated hydroxyl-containing epoxy fatty acid esters, carboxylated polyesters, carboxylated alkyd resins, carboxylated acrylic interpolymers free of amide groups, and carboxylated vinyl interpolymers. While utility in these systems is disclosed, there is no suggestion of the use of reactive urea, thiourea, or carbamate derivatives as reactive diluents in phenolic or urea-aldehyde resin systems.
U.S. Pat. No. 3,862,060 (Anderson) descibes stable emulsions containing high concentrations of thermosettable phenol-formaldehyde resole resins as the dispersed phase. The emulsions exhibit a water tolerance of less than 40 percent, and are stabilized with a proteinaceous compound. The resole resins are prepared with amine catalysts and are preferably modified with melamine. The emulsions optionally contain urea or dicyandiamide, which act to reduce the free formaldehyde.
U.S. Pat. No. 5,008,336 (Richey, Jr., et al.) describes tri-substituted amino oxides having at least two reactive hydroxyls useful as reactive diluents in polyol polymer-containing coating compositions.
U.S. Pat. No. 4,903,440 (Larson, et al.) describes a modified resole phenolic resin for use in abrasive articles containing a binder having at least 1.1 pendant alpha, beta-unsaturated carbonyl groups per molecule but does not suggest the storage life benefits of the compositions described herein, nor the compositions themselves.
U.S. Pat. No. 3,817,976 (Bakul, et al.) discloses the use of a butadiene-nitrile rubber as a modifier, but requires the use of additional organic solvents.
U.S. Pat. No. 4,505,712 (Floyd, et al.) and U.S. Pat. Nos. 4,345,063; 4,285,690; 4,332,586; and 4,284,758 (North) describe the use of cyclic urea derivatives for treating textile fabrics and paper in the absence of formaldehyde, but likewise requires organic solvent additions.
U.S. Pat. No. 4,927,431 (Buchanan, et al.) describes a modified phenolic resin binder for use in abrasive articles which contains a radiation-curable component containing pendant acrylate groups, but requires the addition of organic solvents.
U.S. Pat. No. 5,026,405 (Guerro) describes a modified phenolic resin incorporating certain alkyl or hydroxyalkylcarbamylmethyl triazines to improve the wet strength of bonded abrasive articles. The dry strength of the abrasive articles is diminished, however.
U.S. Pat. No. 4,802,896 (Law, et al.) discloses a modified phenolic resin binder for use in abrasive articles which contains a thermally-stable aromatic ligand. Added organic solvents are required in this composition as well.
U.S. Pat. No. 4,904,516 (Creamer) describes a water soluble phenolic resin containing a soluble alkaline earth metal salt for use in binding glass fiber batts.
Other patents which are of interest include U.S. Pat. Nos. 4,785,073; 4,311,631; 4,515,835; and 4,108,840.
This invention addresses many of the above problems associated with binder precursor solutions which include one or more thermally curable resins (including phenolic resins) which are used as binders in the manufacture of abrasive articles.
As emissions of VOC are increasingly being regulated, an unmet need exists in the art of manufacturing abrasive articles for coatable, thermally curable binder precursor solutions which reduce or substantially eliminate the use of VOC as solvents and which scavenge unreacted resin precursors such as phenol and aldehydes. It would also be advantageous if a coatable, thermally curable binder precursor solution could be developed having increased water tolerance and dry hardness (i.e., cured binder hardness), and gel time comparable to or less than previously known coatable, thermally curable binder precursor solutions, while reducing VOC emissions.